From [Salesforce docs][1]Salesforce docs,
When a custom object is on the detail side of a master-detail relationship with a standard object, its organization-wide default is set to Controlled by Parent and it is not editable.
If you've got a Master-Detail between two custom objects, I think they have their own sharing settings.
EDIT: For custom object Master records, the sharing should apply to the Detail record. Do you have access to "View All Data" like an admin access? [1]: https://help.salesforce.com/articleView?id=admin_sharing.htm&language=en_US&type=0